In recent years, for a mobile device such as a cell phone or a PDA, a cover glass (protective glass) for protecting the display and improving the appearance has been used in many cases. On the other hand, for such a mobile information device, weight reduction and thickness reduction are required. Therefore, a cover glass to be used for display protection is also required to be thin.
However, when the thickness of the cover glass is made to be thin, there has been a problem that the strength is lowered, the cover glass itself is broken by, for example dropping while using or carrying, and the cover glass cannot accomplish the essential role to protect a display device.
In order to solve the above problem, it is conceivable to improve the strength of the cover glass, and as such a method, a method of forming a compressive stress layer on a glass surface is commonly known. The representative method of forming a compressive stress layer on a glass surface is an air-blast quenching strengthening method (physical strengthening method) wherein a glass sheet surface heated to near the softening point is rapidly cooled by air-blast quenching or the like, or a chemical strengthening method wherein alkali metal ions having a small ion radius (typically Li ions or Na ions) at a glass sheet surface are exchanged with alkali ions having a larger ion radius (typically K ions) by ion exchange at a temperature lower than the glass transition point.
As mentioned above, the thickness of the cover glass is required to be thin. However, if the air-blast quenching strengthening method is applied to a thin glass sheet, since a temperature difference between the surface and the inside tends not to arise, it is difficult to form a compressive stress layer and an objective property of high strength cannot be obtained. Therefore, a cover glass strengthened by the latter chemical strengthening method is usually used (see Patent Document 1).
As mentioned above, a chemically strengthened glass obtained by chemically strengthening glass is very effective for protecting the display and improving the appearance in a mobile device such as a cellular phone or a PDA.
Since a usual glass, typically a soda lime silica glass or the like widely used as a window glass contains a minute amount of a coloring component such as Fe or Ti, the glass is colored yellow, green, or blue, unlike plastics, and thus it is difficult to obtain clarity and the appearance is not good. Particularly, in the case of a structure that the cross-section is uncovered, when the cover glass is viewed from the side, the cover glass is colored and the appearance is not good.
Moreover, since an optical path length varies between the case where it is viewed perpendicular to a display and the case where it is viewed laterally relative to the display, there is a problem that a tint varies depending on the angle when the glass is colored as mentioned above.